1. Field of the Invention
This invention relates to a magnetic field detection circuit for detecting the intensity of the external magnetic field by using a magnetic impedance device whose impedance changes as a function of the external magnetic field in a state where a high frequency electric current is applied. More particularly, the present invention relates to a magnetic field detection circuit for highly sensitively and accurately detecting a very weak magnetic field generated by the terrestrial magnetism or a very weak electric current.
2. Related Background Art
In recent years, bearing sensors adapted to detect the terrestrial magnetism and electric current sensors capable of detecting a very weak electric current have come to be required to highly sensitively and accurately detect a very weak magnetic field so as to broaden the scope of application. Magnetic impedance devices (to be referred to as MI devices hereinafter) have been attracting attention as magnetic field detecting devices of this type. With a known magnetic field detecting method using MI devices, the magnetic field is detected by directly applying a high frequency electric current to a magnetic object and detecting the voltage signal generated by the detection coil wound around or arranged in the vicinity of the magnetic object.
The magnetic signal obtained by this detection method is odd-functional relative to an external magnetic field and has an advantage of providing a sufficient level of sensitivity at and near a nil-magnetic field without applying a bias magnetic field.
FIG. 10 of the accompanying drawings is a basic circuit diagram of a circuit that can be used for this magnetic field detection method. Referring to FIG. 10, a pulse oscillation is generated in an MHz band by means of an oscillation circuit 1 formed by C-MOS inverters and a CR circuit surrounded by broken lines and an electric current is made to flow to MI device 4 by way of C-MOS inverter 2 and current regulating resistor 3. Note that the high frequency current (pulse current) generated by this circuit is modulated only at the positive side.
Then, the change in the magnetic flux caused by the MI device 4 is taken out as a change in the voltage generated in detection coil 5 by winding the detection coil 5 around the MI device 4 to form a solenoid or making it turn flat and bringing it close to the MI device 4. One of the opposite ends of the detection coil 5 is grounded while the other end is connected to waveform detection circuit 6 formed by a diode and a CR circuit so that an amplitude-modulated magnetic field signal is taken out from the waveform detection circuit 6. Alternatively, the magnetic field signal may be detected by synchronous detection substantially in synchronism with rises and falls of oscillation of the oscillation circuit 1 by means of an analog switch.
FIG. 11 shows such a magnetic field signal. As shown, the voltage waveform Vw of the detection coil 5 shows peaks corresponding to rises and falls in the waveform of the pulse current waveform Iw flowing to the MI device 4. Plus peaks pp and minus peaks mp are symmetrically arranged relative to the base line and move in opposite directions as indicated by arrows. The amplitude of the peaks varies as a function of that of the external magnetic field (H). The output Vs of the detection circuit is S-shaped as shown in FIG. 12 and shows a linear slope at and near a nil-magnetic field.
Meanwhile, from the viewpoint of application of bearing sensors and electric current sensors, it is essential that the sensor is so set that the signal output is nil when a nil-magnetic field is detected, or so-called nil-point setting is in place, in the operation of detecting the magnetic field of a DC current. If the nil-point setting is not in place, the detection accuracy of the bearing or electric current sensor will be adversely affected to a large extent.
More specifically, in the case of a bearing sensor, the terrestrial magnetism is measured by arranging magnetism detection devices such as MI devices respectively on the X and Y axes that rectangularly intersect each other on a horizontal plane and the azimuth is determined on the basis of the DC output voltages of the devices. A linear sensitivity of the sensor itself relative to an external magnetic field and the stability of the output absolute voltages Vx, Vy are essential to the accuracy of measuring the azimuth. Particularly, the stability of the output voltages has a vital importance.
The azimuth is determined by means of formula θ=tan−1 {(Vx−Vxo)/(Vy−Vyo)}. However, if the outputs Vxo, Vyo for a nil-magnetic field are not accurate, the calculation using the formula inevitably comes to involve errors. Particularly, the horizontal component of the terrestrial magnetism can be less than 100 mG (milligausses) at a place close to either of the magnetic poles or in a building so that the accuracy of calculation can be significantly affected if the outputs for a nil-magnetic field involve an error corresponding to 10 mG.
In the case of an electric current sensor, again, if the nil-point setting is not reliable, there can easily arise an error of several milliamperes (mA) in response to an output error of several milligausses (mG) when transforming the magnetic field from an electric current line into a voltage and evaluating the electric current that may be a DC as weak as tens of several milliamperes (mA).
Therefore, with the known arrangement of FIG. 10, the voltage Vso for a nil-magnetic field is detected in the characteristic graph of FIG. 12 and a reference voltage that matches the voltage Vso is selected by means of an amplifier 7 having a variable resistor 7a inserted between the power supply voltage and the grounding terminal. Then, the nil-point voltage is regulated manually, seeing the output of the amplifier.
However, the sensitivity can change as the ambient temperature changes. Then, the characteristic curve can be shifted in a manner as indicated by a broken line in FIG. 12. Under such circumstances, it is difficult to manually regulate the nil-point voltage. While it is theoretically possible to incorporate a circuit for automatically regulating the nil-point voltage, the overall circuit configuration will become highly complex and the cost will become prohibitive.